Various kinds of apparatus are used for preparing coffee, more particularly espresso coffee--i.e., an infusion produced with hot water and coffee powder--and in all of them water at a high temperature and an appropriate pressure passes through a layer of coffee powder, the resulting infusion then being collected.
To produce a high-quality coffee the infusion water must be at a temperature below the boiling point in order to obviate removing from the coffee powder not only the aromatic ingredients which give the beverage its taste, but also undesirable and in some cases harmful ingredients which are evolved from the coffee powder only at temperatures above the boiling point of water.
Consequently, apparatuses in which water passes through the coffee powder at a temperature below the boiling point and which therefore yield a high-quality product are very widely used both commercially and privately.
More particularly, for domestic use a compact and low-cost coffee-making machine is required.
In automatic coffee-making machines the coffee powder is disposed in a filter carrier having a filter for retaining the coffee powder and discharging the infusion produced. A piston compresses the powder to an appropriate extent and hot water is supplied by way of the piston to produce the infusion. A driving element is therefore necessary to introduce the piston into the filter carrier and to retain the piston thus positioned while hot pressurized water is supplied.
Operating units using irreversible geared motors can be used for this purpose but such devices are expensive and are unsuitable for a domestic machine. Similar problems arise in connection with the other operating steps required, such as metering of the water, control of the cycle and so on.